Heat your oven to 180°C/Fan 160°C/Gas 4. Tip the flour into a food processor and scoop out 1 tbsp for later. Add 1 tsp baking powder and a pinch of salt. Chop in the butter. Blitz to make a fine, sandytextured mixture. No processor? Combine everything in a bowl and rub together with your fingertips to make breadcrumbs.
Add 3-4 tbsp cold water and briefly blitz to bring the pastry together (if you're not using a processor, stir the water in till the pastry comes together). Tip the pastry out onto your work surface. Press it into a flat round. Wrap in cling film or foil and pop it in the freezer to chill.
Peel and finely slice the shallots. Peel and finely chop the carrots. Trim the celery and finely slice it. Finely slice the sorrel. Slice the portobello mushrooms. Chop the chicken breast mini fillets into bite-size pieces.
Warm a medium pan for 1 min, then add ½ tbsp oil and the chicken. Fry for 5 mins, turning the chicken pieces once or twice, till they're browned all over. Scoop out of the pan and pop on a plate.
Add the shallots, carrots, mushrooms and celery to the pan with a splash of water. Season with salt and pepper. Keep the heat low and cook, stirring occasionally, for 5 mins till the veg are soft.
Stir the chicken back into the pan with any juices from the plate. Add the sorrel and 1 tbsp mustard. Crumble in the stock cube. Stir in the cream. Taste and season with more salt and pepper if you think it needs it. Spoon the mix into a 1 ltr ovenproof dish or 2 x 500ml dishes.
Dust your work surface with the flour you saved in step 1. Roll out the pastry to make a lid to cover the dish. Brush the edges of the pastry with water and then press them onto the rim of the dish to seal the edges. Poke a hole in the middle of the lid. Brush with water. Bake for 30 mins till browned.
While the pie cooks, shred half the cabbage. Bring a pan of salted water to the boil. When it's boiling (and the pie is almost ready), add the cabbage to the pan. Simmer for 5 mins or till cooked. Drain well. Serve the pie in warm bowls with the cabbage on the side.